The present invention relates to an inexpensive printed wiring board having a high wiring density as well as to a process for producing such a wiring board efficiently.
Printed wiring boards have been produced by a subtractive process which comprises drilling holes at the positions of a copper-clad laminate requiring electrical connection, coating the walls of the holes with a metal by electroless plating or electroplating, and removing the unnecessary portions of the conductor (copper) by etching; an additive process which comprises drilling holes in an insulating substrate and applying electroless plating to the walls of the holes as well as to the necessary parts of the substrate surface to form conductor circuits; a semiadditive process which comprises drilling holes in the same manner as in the subtractive process, then removing the unnecessary portions of the copper foil, and coating only the walls of the holes with a metal by electroless plating; and processes similar thereto.
Meanwhile, in order to produce a wiring board inexpensively by forming circuits without applying plating, it has long been conducted to coat the walls of the holes with a silver or copper paste comprising particles of said metal, a binder and a solvent, as shown in FIG. 2, or to form circuits with said paste per se. It is also known to coat the walls of the holes with a solder paste and melt the paste to complete connection.
Of the above processes for production of a wiring board, those using plating have been used for production of a wiring board of high connection reliability. With the recent spread of electronic appliances, it is strongly required to provide electronic appliances of high performance with low cost.
In the above processes for wiring board production using plating, the time for plating cannot be made shorter than now and a fairly large apparatus is required because particular apparatuses for plating are required and the plating solution must be controlled.
Thus, under the current situation, the efficiency of wiring board production has a limitation as long as plating is employed.
In the processes for producing a wiring board inexpensively, as shown in FIG. 2, the coating of an electroconductive paste is unable to achieve the complete filling of the hole inside; the connection area is small; the connection resistance is large; as a result, only a wiring board of small wiring density such as one-side circuit board or two-side-circuit board is obtained. In the above process applying an electroconductive paste by printing, it is impossible to form complicated circuits and the connection reliability is too low to produce a multilayer wiring board. Thus, it is difficult to produce a wiring board of high performance using inexpensive materials.